FIG. 1 illustrates a typical prior art wet-type friction engagement device 10 having friction material on both sides. Friction engagement device 10 comprises internal tooth discs 14 which are spline-fitted in a hub 12 and external tooth discs 18 which are spline-fitted in a drum 16 and are alternated with the internal tooth discs 14.
Each of the internal tooth discs 14 is fixedly provided with friction material 15 on both surfaces thereof and, when a hydraulically operated piston 11 is moved in an axial direction, relative to discs 14 and 18, the internal tooth discs 14 are engaged with the external tooth discs 18 to transmit torque. In prior art wet-type friction engagement device 10, cooling oil is supplied to the friction sliding surfaces.
Typically, friction material 15 is fixed to internal tooth disc 14 so that friction engagement device 10 comprises high torque transmission capacity and is durable enough to withstand the high temperature produced by sliding friction during actuation.
Prior art friction material 15 typically has a large static coefficient of friction and a high torque transmission capacity. However, friction material having a large static coefficient of friction is inferior in heat resistance. Thus, in prior art friction engagement devices 10, friction materials having a large static coefficient of friction, are formed such that they have a porous texture or structure having an intricate network of pores that are open on its surface and adapted to hold oil and thereby cool the friction material during engagement to improve heat resistance.
Based on the prior art alone, it would appear that highly porous friction material can hold more oil and therefore can improve cooling effectiveness and hence can improve heat resistance. However, as shown in FIG. 2, if the degree of porosity of the friction material is increased so as to improve heat resistance, transmission torque output by a friction engagement device 10, at the beginning of engagement, is increased, and its torque characteristic varies with time as compared with a friction material having a low degree of porosity. FIG. 3 shows the relationship between oil penetrant time (X axis) during which oil on a sliding surface penetrates into a friction material, and the dynamic coefficient of friction .mu.i (Y axis) at the beginning of engagement for a plurality of friction materials having different degrees of porosity (and hence being different in penetrant time).
As is evident from FIGS. 2 and 3, if the friction material has a high degree of porosity, the thickness of the film of oil disposed between the friction material and a plate associated therewith is difficult to maintain on the sliding surface, and quickly penetrates into the friction material thereby reducing the thickness of the oil film on the sliding surface. As a result, the dynamic coefficient of friction .mu.i increases at the beginning of engagement. In other words, if friction material having a high degree of porosity is used so as to improve heat resistance in a typical wet-type friction engagement device 10, its dynamic coefficient of friction increases. Additionally, friction material having a high degree of porosity exhibits the problem of time-varying torque transmission at the beginning of engagement. These time variations in transmission of the torque are manifested as a speed change shock in the system.